Workholding apparatuses, or workholders, have been used for a number of years and are useful for holding solid and annular workpieces for subsequent machining or grinding. As is known, workholding apparatuses may be mechanically actuated or hydraulically actuated. In certain applications, workpieces formed from cast material have relatively non-uniform surfaces and significant dimensional variances in both inside and outside diameters. These workpiece variances may result in poor circumferential surface contact between the non-uniform workpieces and the smooth metal collet of the workholder. Furthermore, such workpiece size and shape variances often require large amounts of workholder expansion in order to achieve an adequate grip.
One workholding solution employs a circumferentially continuous steel sleeve to grip the workpiece. This solution requires relatively high hydraulic pressure on a workholder to achieve a suitable grip on the workpiece. For example, over 6,000 PSI of fluid pressure is required to achieve 0.001″ expansion of a typical one-inch diameter steel sleeve. This result is achieved by increasing hydraulic-system size and hydraulic-system fittings, thereby increasing implementation costs. As a result of the high loads and pressures, permanent, non-rebounding indentations are left in the steel sleeve, thereby accelerating its replacement. Unfortunately, the high pressure can generate greater wear and failure of polymeric O-rings, possibly leading to fluid leaks. Also, a steel sleeve is expandable only a limited amount before being over expanded beyond a non-recoverable elastic limit of the sleeve.
Another common workholding solution relies on longitudinal slots cut out of, and circumferentially spaced about, a workholder sleeve. An exemplary apparatus comprises a hydrostatic workholder having a body partially defining a fluid chamber and carrying a deflectable metal sleeve formed with a plurality of circumferentially spaced and longitudinally extending slots. The apparatus also includes a flexible, polymeric bladder partially defining the fluid chamber to separate the fluid in the chamber from the sleeve. The longitudinal slots allow greater expansion of the sleeve and, thus, a more robust fit to parts with significant surface variances. But these sleeve openings allow foreign matter to enter the workholder, thereby increasing service frequency and device failure rates. And, the openings also create a weaker sleeve compared to solid sleeve workholders of similar thicknesses. Finally, under sufficiently high fluid pressures, the bladder may extrude through the openings in the sleeve.
Sleeves are most commonly fashioned from steel resulting in high durability after repeated use. But to achieve the optimum fit between workpiece and workholder, sleeves composed of metal-coated plastic have been implemented. This design provides a closer nexus between workpiece and workholder. But after many uses in high-performance operations, a decrease in gripping pressure can result. Moreover, while plastic sleeves are effective at gripping a workpiece, after frequent deflections, the ability of plastic to maintain its original shape decreases over time.